In English there are four basic conditional structures, although these can be mixed according to the situation. They can also be mixed in other languages.
The tenses I've used are the BASIC tenses. However, once you know the general structures you can check what other verb forms you can use.
This is used when there is no condition, in other words you could substitute 'if' with 'when'. It is often used when describing facts or explaining how something works.
Structure: if + present, present
The first conditional is used for situations based on fact. The condition describes something normal and possible, and the result is probable and based on the present or the future.
Structure: if + present, future
This conditional is not based on fact. It refers to a situation in the present or future which is unreal, unlikely or contrary to facts.
To show this unreality, we have to shift the tense from the present to the past, although the condition still refers to the present or the future.
Structure: if + past, would + infinitive
The third conditional refers to situations in the past which, because they're in the past, are imaginary or impossible. You can't change the past.
Structure: if + past perfect, would (could/might) have + past participle
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